I. Field of the Invention
The present invention relates to the field of flat lamps and in particular it relates to flat coplanar-discharge lamps and to the use of these lamps.
II. Description of Related Art
As is known, flat lamps, used for the manufacture of back-lit liquid-crystal display (LCD) devices or as decorative or architectural luminaires, consist of two glass sheets kept apart by a short distance, generally less than a few millimeters, and hermetically sealed. The internal space contains a gas under reduced pressure, which emits ultraviolet (UV) radiation which excites a phosphor material emitting in the visible and covering the internal faces of the glass plates.
UV lamps are also formed by choosing a glass that transmits the UV radiation from the emitting gas or a phosphor material emitting in the UV.
In a conventional flat lamp structure, one of the glass sheets also has, on its internal face, electrodes mainly in the form of mutually parallel conducting bands. At a given instant, two adjacent electrodes constitute a cathode and an anode, between which what is called a coplanar discharge is produced, that is to say a discharge in a direction hugging the main surface of the supporting glass sheet.
To supply this coplanar discharge, a high-frequency voltage source is used that delivers a train of pulses with a short rise time, usually rectangular pulses.
It is also accepted that this coplanar discharge is homogeneous (i.e. filament-free) only with a duty cycle, (corresponding to the ratio of the conduction time to the period of the pulse train) that is very short, around 4%, which is technically complicated to achieve and consequently expensive.
To guarantee the homogeneity of the radiation from a conventional lamp, with a pulse train having a longer duty cycle, it will be necessary to combine an optical diffuser with the emitting surface. Here again, this complicates the production of the flat lamp. What is more, the thickness is increased, as is the weight. Furthermore, this solution cannot be easily transposed to UV lamps.